package org.apache.lucene.search.spans;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import org.apache.lucene.index.IndexReader;
import org.apache.lucene.util.ArrayUtil;

import java.io.IOException;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Collection;
import java.util.Set;

/** A Spans that is formed from the ordered subspans of a SpanNearQuery
 * where the subspans do not overlap and have a maximum slop between them.
 * <p>
 * The formed spans only contains minimum slop matches.<br>
 * The matching slop is computed from the distance(s) between
 * the non overlapping matching Spans.<br>
 * Successive matches are always formed from the successive Spans
 * of the SpanNearQuery.
 * <p>
 * The formed spans may contain overlaps when the slop is at least 1.
 * For example, when querying using
 * <pre>t1 t2 t3</pre>
 * with slop at least 1, the fragment:
 * <pre>t1 t2 t1 t3 t2 t3</pre>
 * matches twice:
 * <pre>t1 t2 .. t3      </pre>
 * <pre>      t1 .. t2 t3</pre>
 *
 *
 * Expert:
 * Only public for subclassing.  Most implementations should not need this class
 */
public class NearSpansOrdered extends Spans {
    private final int allowedSlop;
    private boolean firstTime = true;
    private boolean more = false;

    /** The spans in the same order as the SpanNearQuery */
    private final Spans[] subSpans;

    /** Indicates that all subSpans have same doc() */
    private boolean inSameDoc = false;

    private int matchDoc = -1;
    private int matchStart = -1;
    private int matchEnd = -1;
    private List<byte[]> matchPayload;

    private final Spans[] subSpansByDoc;
    private final Comparator<Spans> spanDocComparator = new Comparator<Spans>() {
        public int compare(Spans o1, Spans o2) {
            return o1.doc() - o2.doc();
        }
    };

    private SpanNearQuery query;
    private boolean collectPayloads = true;

    public NearSpansOrdered(SpanNearQuery spanNearQuery, IndexReader reader) throws IOException {
        this(spanNearQuery, reader, true);
    }

    public NearSpansOrdered(SpanNearQuery spanNearQuery, IndexReader reader, boolean collectPayloads) throws IOException {
        if (spanNearQuery.getClauses().length < 2) {
            throw new IllegalArgumentException("Less than 2 clauses: " + spanNearQuery);
        }
        this.collectPayloads = collectPayloads;
        allowedSlop = spanNearQuery.getSlop();
        SpanQuery[] clauses = spanNearQuery.getClauses();
        subSpans = new Spans[clauses.length];
        matchPayload = new LinkedList<byte[]>();
        subSpansByDoc = new Spans[clauses.length];
        for (int i = 0; i < clauses.length; i++) {
            subSpans[i] = clauses[i].getSpans(reader);
            subSpansByDoc[i] = subSpans[i]; // used in toSameDoc()
        }
        query = spanNearQuery; // kept for toString() only.
    }

    // inherit javadocs
    @Override
    public int doc() {
        return matchDoc;
    }

    // inherit javadocs
    @Override
    public int start() {
        return matchStart;
    }

    // inherit javadocs
    @Override
    public int end() {
        return matchEnd;
    }

    public Spans[] getSubSpans() {
        return subSpans;
    }

    // TODO: Remove warning after API has been finalized
    // TODO: Would be nice to be able to lazy load payloads
    @Override
    public Collection<byte[]> getPayload() throws IOException {
        return matchPayload;
    }

    // TODO: Remove warning after API has been finalized
    @Override
    public boolean isPayloadAvailable() {
        return matchPayload.isEmpty() == false;
    }

    // inherit javadocs
    @Override
    public boolean next() throws IOException {
        if (firstTime) {
            firstTime = false;
            for (int i = 0; i < subSpans.length; i++) {
                if (!subSpans[i].next()) {
                    more = false;
                    return false;
                }
            }
            more = true;
        }
        if (collectPayloads) {
            matchPayload.clear();
        }
        return advanceAfterOrdered();
    }

    // inherit javadocs
    @Override
    public boolean skipTo(int target) throws IOException {
        if (firstTime) {
            firstTime = false;
            for (int i = 0; i < subSpans.length; i++) {
                if (!subSpans[i].skipTo(target)) {
                    more = false;
                    return false;
                }
            }
            more = true;
        } else if (more && (subSpans[0].doc() < target)) {
            if (subSpans[0].skipTo(target)) {
                inSameDoc = false;
            } else {
                more = false;
                return false;
            }
        }
        if (collectPayloads) {
            matchPayload.clear();
        }
        return advanceAfterOrdered();
    }

    /** Advances the subSpans to just after an ordered match with a minimum slop
     * that is smaller than the slop allowed by the SpanNearQuery.
     * @return true iff there is such a match.
     */
    private boolean advanceAfterOrdered() throws IOException {
        while (more && (inSameDoc || toSameDoc())) {
            if (stretchToOrder() && shrinkToAfterShortestMatch()) {
                return true;
            }
        }
        return false; // no more matches
    }

    /** Advance the subSpans to the same document */
    private boolean toSameDoc() throws IOException {
        ArrayUtil.mergeSort(subSpansByDoc, spanDocComparator);
        int firstIndex = 0;
        int maxDoc = subSpansByDoc[subSpansByDoc.length - 1].doc();
        while (subSpansByDoc[firstIndex].doc() != maxDoc) {
            if (!subSpansByDoc[firstIndex].skipTo(maxDoc)) {
                more = false;
                inSameDoc = false;
                return false;
            }
            maxDoc = subSpansByDoc[firstIndex].doc();
            if (++firstIndex == subSpansByDoc.length) {
                firstIndex = 0;
            }
        }
        for (int i = 0; i < subSpansByDoc.length; i++) {
            assert (subSpansByDoc[i].doc() == maxDoc) : " NearSpansOrdered.toSameDoc() spans " + subSpansByDoc[0] + "\n at doc " + subSpansByDoc[i].doc() + ", but should be at " + maxDoc;
        }
        inSameDoc = true;
        return true;
    }

    /** Check whether two Spans in the same document are ordered.
     * @param spans1 
     * @param spans2 
     * @return true iff spans1 starts before spans2
     *              or the spans start at the same position,
     *              and spans1 ends before spans2.
     */
    static final boolean docSpansOrdered(Spans spans1, Spans spans2) {
        assert spans1.doc() == spans2.doc() : "doc1 " + spans1.doc() + " != doc2 " + spans2.doc();
        int start1 = spans1.start();
        int start2 = spans2.start();
        /* Do not call docSpansOrdered(int,int,int,int) to avoid invoking .end() : */
        return (start1 == start2) ? (spans1.end() < spans2.end()) : (start1 < start2);
    }

    /** Like {@link #docSpansOrdered(Spans,Spans)}, but use the spans
     * starts and ends as parameters.
     */
    private static final boolean docSpansOrdered(int start1, int end1, int start2, int end2) {
        return (start1 == start2) ? (end1 < end2) : (start1 < start2);
    }

    /** Order the subSpans within the same document by advancing all later spans
     * after the previous one.
     */
    private boolean stretchToOrder() throws IOException {
        matchDoc = subSpans[0].doc();
        for (int i = 1; inSameDoc && (i < subSpans.length); i++) {
            while (!docSpansOrdered(subSpans[i - 1], subSpans[i])) {
                if (!subSpans[i].next()) {
                    inSameDoc = false;
                    more = false;
                    break;
                } else if (matchDoc != subSpans[i].doc()) {
                    inSameDoc = false;
                    break;
                }
            }
        }
        return inSameDoc;
    }

    /** The subSpans are ordered in the same doc, so there is a possible match.
     * Compute the slop while making the match as short as possible by advancing
     * all subSpans except the last one in reverse order.
     */
    private boolean shrinkToAfterShortestMatch() throws IOException {
        matchStart = subSpans[subSpans.length - 1].start();
        matchEnd = subSpans[subSpans.length - 1].end();
        Set<byte[]> possibleMatchPayloads = new HashSet<byte[]>();
        if (subSpans[subSpans.length - 1].isPayloadAvailable()) {
            possibleMatchPayloads.addAll(subSpans[subSpans.length - 1].getPayload());
        }

        Collection<byte[]> possiblePayload = null;

        int matchSlop = 0;
        int lastStart = matchStart;
        int lastEnd = matchEnd;
        for (int i = subSpans.length - 2; i >= 0; i--) {
            Spans prevSpans = subSpans[i];
            if (collectPayloads && prevSpans.isPayloadAvailable()) {
                Collection<byte[]> payload = prevSpans.getPayload();
                possiblePayload = new ArrayList<byte[]>(payload.size());
                possiblePayload.addAll(payload);
            }

            int prevStart = prevSpans.start();
            int prevEnd = prevSpans.end();
            while (true) { // Advance prevSpans until after (lastStart, lastEnd)
                if (!prevSpans.next()) {
                    inSameDoc = false;
                    more = false;
                    break; // Check remaining subSpans for final match.
                } else if (matchDoc != prevSpans.doc()) {
                    inSameDoc = false; // The last subSpans is not advanced here.
                    break; // Check remaining subSpans for last match in this document.
                } else {
                    int ppStart = prevSpans.start();
                    int ppEnd = prevSpans.end(); // Cannot avoid invoking .end()
                    if (!docSpansOrdered(ppStart, ppEnd, lastStart, lastEnd)) {
                        break; // Check remaining subSpans.
                    } else { // prevSpans still before (lastStart, lastEnd)
                        prevStart = ppStart;
                        prevEnd = ppEnd;
                        if (collectPayloads && prevSpans.isPayloadAvailable()) {
                            Collection<byte[]> payload = prevSpans.getPayload();
                            possiblePayload = new ArrayList<byte[]>(payload.size());
                            possiblePayload.addAll(payload);
                        }
                    }
                }
            }

            if (collectPayloads && possiblePayload != null) {
                possibleMatchPayloads.addAll(possiblePayload);
            }

            assert prevStart <= matchStart;
            if (matchStart > prevEnd) { // Only non overlapping spans add to slop.
                matchSlop += (matchStart - prevEnd);
            }

            /* Do not break on (matchSlop > allowedSlop) here to make sure
             * that subSpans[0] is advanced after the match, if any.
             */
            matchStart = prevStart;
            lastStart = prevStart;
            lastEnd = prevEnd;
        }

        boolean match = matchSlop <= allowedSlop;

        if (collectPayloads && match && possibleMatchPayloads.size() > 0) {
            matchPayload.addAll(possibleMatchPayloads);
        }

        return match; // ordered and allowed slop
    }

    @Override
    public String toString() {
        return getClass().getName() + "(" + query.toString() + ")@" + (firstTime ? "START" : (more ? (doc() + ":" + start() + "-" + end()) : "END"));
    }
}
